1. Field of the Invention
The subject invention relates to vibration damping apparatus and, more specifically, to apparatus including dynamic damping systems and apparatus for damping vibrations in vibration sensitive equipment, as well as apparatus for securing vibration sensitive equipment relative to a support.
2. Description of the Prior Art
For a number of years, rods or posts have been employed to position components in optical systems. These rods are typically rigidly affixed to a bench, table or other common support, while the components, such as lenses, prisms, mirrors, or other apparatus mounted in holding devices, are secured to the rod via collars or sleeves incorporated in the component assembly. These are convenient arrangements since they enable continuous positioning of the component along the length and possibly around the axis of the rod and consequently the adjustment of the relative positions of optical components in a system. By suitable design of the interface between the component assembly and the rod, and by the use of locking screws, collar clamps or other fastening devices, any looseness between the rod and component assembly can be eliminated and the rod and component assembly behave as a single rigid body affixed to the common support.
It is most desirable and in some cases mandatory that relative motions between several different rod-mounted components be held to very small values, indeed to less than 10.sup.-6 inches, in order to apply certain optical techniques such as holography or interferometry. For such small values of relative motion, the response of the rods to ambient vibrations and airborne disturbances becomes a major concern and it is necessary to employ rods of appreciable cross section and great inherent static rigidity. However, when such rods are constructed of metal or other materials possessing the large elastic modulus necessary to provide this great inherent static rigidity in a reasonable volume, a serious problem reamins. Such materials characteristically lack sufficient inherent structural damping to avoid large resonant motions of the rod assembly when the frequency of any ambient excitation corresponds with one of the structural resonances of the rod assembly. When this occurs, the rod assembly exhibits large motions in response to quite small excitation levels and may consequently prevent successful operation of the optical system. Therefore, it is most desirable to provide some additional means of damping rod structures for such applications, which means should ideally emulate intrinsic structural damping.
Most prior art devices of any relevance are of the shock mount or vibration isolation type, as may be seen from U.S. Pat. Nos. 2,642,252, 2,819,060, 2,948,126, 3,128,978, 3,181,850, 3,305,227, 3,430,902 and 3,764,100. Devices of this kind would be unsuitable for present purposes, since they trap support-coupled vibrational energy in the support structure and possess inadequate static rigidity. The remaining prior-art damping devices may be identified in terms of four distinct types of damping structures, treated in the extensive engineering literature on the subject (see G. H. Bruns and A. D. Nashif, "Experimental Verification of Theory of Damping of a Simple Structure by Distributed Tuned Dampers," Air Force Materials Laboratory, Wright-Patterson Air Force Base Report No. AFML-TR-65-440 [Jan. 1966]; J.C. Snowdon, "Vibration and Shock in Damped Mechanical Systems,"[Wiley, New York, 1968] Ch. 10; Harris and Crede, "Shock and Vibration Handbook" [McGraw Hill, New York, 1961] Col. 1, Ch. 6).
One of these four types may be termed constrained layer damping and comprises an arrangement in which a relatively thin layer of lossy viscoelastic material is confined or bonded between two layers of structural material, either or both of which may be rigidly attached to the main structure in such a way that flexure of the structure during vibration is coupled to one or both of the structural members and results in energy dissipating cyclic shear stresses in the viscoelastic layer. While simple in concept and intrinsically broadband, this method does not lend itself to ready incorporation in a rodlike structure, nor, more significantly, does it provide the large requisite damping. The proposal of U.S. Pat. No. 3,314,502, by R. P. Thorn, issued Apr. 18, 1967, falls into this class.
Another of the four types, the undamped dynamic absorber, consists of an undamped auxiliary mass, spring coupled to the principal structure, and tuned to the typically constant frequency of excitation of the principal structure so that at this frequency the absorber enforces a motional node at its point of attachment. This arrangement has a very narrow effective bandwidth and requires either a relatively large absorber mass or excessively large absorber motional amplitudes. It is typically employed with rotating machinery. The basic proposal contained in U.S. Pat. No. 2,268,495, by O. S. Petty, may be considered in this context.
A third type, the untuned viscous damper, consists of an auxiliary mass statically uncoupled from the main structure (i.e., not tending to recover from slow displacement relative to the main structure) and yet having viscous damping of motions relative to the main structure. This arrangement has the same broadband, moderate damping characteristics as the constrained layer arrangement and moreover is ineffective for small motional amplitudes. The proposal of British Patent Specification 829,562 falls into this category.
The fourth type, known as tuned, viscously damped dyanamic absorber, consists of an auxiliary mass, sometimes called seismic mass, spring coupled to the principal structure and possessing damping in its motion relative to the principal structure. This arrangement has an effective frequency bandwidth which is relatively narrow and directly related to the degree of damping and motional amplitude of the auxiliary or seismic mass.